Publications


Comparison between x-ray scattering and velocity-interferometry measurements from shocked liquid deuterium

Physical Review E - Statistical, Nonlinear, and Soft Matter Physics 87 (2013)

K Falk, BJB Crowley, CD Murphy, JS Wark, G Gregori, SP Regan, SX Hu, PB Radha, J Vorberger, DO Gericke, SH Glenzer, AP Jephcoat

The equation of state of light elements is essential to understand the structure of Jovian planets and inertial confinement fusion research. The Omega laser was used to drive a planar shock wave in the cryogenically cooled deuterium, creating warm dense matter conditions. X-ray scattering was used to determine the spectrum near the boundary of the collective and noncollective scattering regimes using a narrow band x-ray source in backscattering geometry. Our scattering spectra are thus sensitive to the individual electron motion as well as the collective plasma behavior and provide a measurement of the electron density, temperature, and ionization state. Our data are consistent with velocity-interferometry measurements previously taken on the same shocked deuterium conditions and presented by K. Falk. This work presents a comparison of the two diagnostic systems and offers a detailed discussion of challenges encountered. ©2013 American Physical Society.


Dynamics for Galactic Archaeology

ArXiv (2013)

J Binney

Our Galaxy is a complex machine in which several processes operate simultaneously: metal-poor gas is accreted, is chemically enriched by dying stars, and then drifts inwards, surrendering its angular momentum to stars; new stars are formed on nearly circular orbits in the equatorial plane and then diffuse through orbit space to eccentric and inclined orbits; the central stellar bar surrenders angular momentum to the surrounding disc and dark halo while acquiring angular momentum from inspiralling gas; the outer parts of the disc are constantly disturbed by satellite objects, both luminous and dark, as they sweep through pericentre. We review the conceptual tools required to bring these complex happenings into focus. Our first concern must be the construction of equilibrium models of the Galaxy, for upon these hang our hopes of determining the Galaxy's mean gravitational field, which is required for every subsequent step. Ideally our equilibrium model should be formulated so that the secular evolution of the system can be modelled with perturbation theory. Such theory can be used to understand how stars diffuse through orbit space from either the thin gas disc in which we presume disc stars formed, or the debris of an accreted object, the presumed origin of many halo stars. Coupling this understanding to the still very uncertain predictions of the theory of stellar evolution and nucleosynthesis, we can finally extract a complete model of the chemodynamic evolution of our reasonably generic Galaxy. We discuss the relation of such a model to cosmological simulations of galaxy formation, which provide general guidance but cannot be relied on for quantitative detail.


Gamma-ray emission in near critical density plasmas

Plasma Physics and Controlled Fusion 55 (2013)

CS Brady, TD Arber, CP Ridgers, AR Bell

Previous work on the interaction of high power lasers with high density targets have identified that emission is primarily from interaction within the skin layer at the target front (e.g. Ridgers et al 2012 Phys. Rev. Lett. 108 165006). This mechanism is inefficient when compared to Reinjected Electron Synchrotron Emission (RESE) for laser interaction with low density solids (Brady et al 2012 Phys. Rev. Lett. 109 245006). However, these detailed analyses of the emission mechanisms were mainly based on 1D simulations and so did not incorporate some important 2D effects. In this paper these 1D emission mechanisms are confirmed to still exist in 2D with comparable properties and a new, intrinsically 2D, emission mechanism, termed edgeglow, is described which can convert 4-5% of the laser energy into gamma-ray energy. © 2013 IOP Publishing Ltd.


Overview of physics results from MAST towards ITER/DEMO and the MAST Upgrade

Nuclear Fusion 53 (2013)

H Meyer, RJ Akers, SY Allan, LC Appel, N Ben Ayed, CD Challis, IT Chapman, D Ciric, G Colyer, NJ Conway, M Cox, BJ Crowley, SC Cowley, G Cunningham, A Darke, RO Dendy, D Dickinson, MD Driscoll, M Dunstan, S Elmore, AR Field, G Fishpool, S Freethy, W Fundamenski, L Garzotti, YC Ghim, MP Gryaznevich, J Harrison, E Havlíčková, NC Hawkes, TC Hender, DF Howell, D Keeling, A Kirk, YQ Liu, B Lloyd, GP Maddison, J Mailloux, R Martin, GJ McArdle, KG McClements, C Michael, F Militello, AW Morris, DG Muir, G Naylor, MR O'Brien, A Patel, SD Pinches, MN Price, CM Roach, M Romanelli, S Saarelma, R Scannell, SE Sharapov, V Shevchenko, S Shibaev, D Stork, J Storrs, A Sykes, D Taylor, N Thomas-Davies, A Thornton, MR Turnyanskiy, M Valovič, G Voss, SEV Warder, IG Abel, M Barnes, E Highcock, P Hill, FI Parra, AA Schekochihin, A Allan, NC Barratt, P Denner, B Dudson, KJ Gibson, K Imada, T O'Gorman, GJ Tallents, RGL Vann, HR Wilson, O Asunta, JW Bradley, J Canik, J Seidl, P Cahyna, J Horáček, S Sangaroon, I Wodniak, M Cecconello, A Danilov, AY Dnestrovsky, Y Dnestrovsky, MFM De Bock, G De Temmerman, T Morgan, D Dunai, S Zoletnik, P Dura, D Higgins, B Hnat, RJ Lake, B McMillan, JR Robinson, E Verwichte, WW Heidbrink, MJ Hole, O Jones, E Kaveeva, P Molchanov, V Rozhansky, P Voskoboynikov, M Kočan, M Lehnen, Y Liang, HJ Leggate, MK Lilley, D Temple, SW Lisgo, S Mordijck, E Nardon, P Tamain, V Naulin, AH Nielsen, S Pamela, A Saveliev, R Zagôrski

New diagnostic, modelling and plant capability on the Mega Ampère Spherical Tokamak (MAST) have delivered important results in key areas for ITER/DEMO and the upcoming MAST Upgrade, a step towards future ST devices on the path to fusion currently under procurement. Micro-stability analysis of the pedestal highlights the potential roles of micro-tearing modes and kinetic ballooning modes for the pedestal formation. Mitigation of edge localized modes (ELM) using resonant magnetic perturbation has been demonstrated for toroidal mode numbers n = 3, 4, 6 with an ELM frequency increase by up to a factor of 9, compatible with pellet fuelling. The peak heat flux of mitigated and natural ELMs follows the same linear trend with ELM energy loss and the first ELM-resolved T measurements in the divertor region are shown. Measurements of flow shear and turbulence dynamics during L-H transitions show filaments erupting from the plasma edge whilst the full flow shear is still present. Off-axis neutral beam injection helps to strongly reduce the redistribution of fast-ions due to fishbone modes when compared to on-axis injection. Low-k ion-scale turbulence has been measured in L-mode and compared to global gyro-kinetic simulations. A statistical analysis of principal turbulence time scales shows them to be of comparable magnitude and reasonably correlated with turbulence decorrelation time. T inside the island of a neoclassical tearing mode allow the analysis of the island evolution without assuming specific models for the heat flux. Other results include the discrepancy of the current profile evolution during the current ramp-up with solutions of the poloidal field diffusion equation, studies of the anomalous Doppler resonance compressional Alfvén eigenmodes, disruption mitigation studies and modelling of the new divertor design for MAST Upgrade. The novel 3D electron Bernstein synthetic imaging shows promising first data sensitive to the edge current profile and flows. © 2013 IAEA, Vienna.


Galactic kinematics and dynamics from RAVE stars

ArXiv (2013)

J Binney, B Burnett, G Kordopatis, M Steinmetz, G Gilmore, O Bienayme, J Bland-Hawthorn, B Famaey, EK Grebel, A Helmi, J Navarro, Q Parker, WA Reid, G Seabroke, F Watson, MEK Williams, RFG Wyse, T Zwitter

We analyse the kinematics of ~400000 RAVE stars. We split the sample into hot and cold dwarfs, red-clump and non-clump giants. The kinematics of the clump giants are consistent with being identical with those of non-clump giants. We fit Gaussian velocity ellipsoids to the meridional-plane components of velocity of each star class and give formulae from which the shape and orientation of the velocity ellipsoid can be determined at any location. The data are consistent with the giants and the cool dwarfs sharing the same velocity ellipsoids; sigma_z rises from 21 kms in the plane to sim 55 kms at |z|=2 kpc, while sigma_r rises from 37 kms to 82 kms. At (R,z) the longest axis of one of these velocity ellipsoids is inclined to the Galactic plane by an angle ~0.8 arctan(z/R). We use a novel formula to obtain precise fits to the highly non-Gaussian distributions of v_phi components. We compare the observed velocity distributions with the predictions of a dynamical model fitted to the velocities of stars that lie within ~150 pc of the Sun and star counts towards the Galactic pole. The model accurately reproduces the non-Gaussian nature of the v_r and v_z distributions and provides excellent fits to the data for v_z at all locations. The model v_phi distributions for the cool dwarfs fit the data extremely well, while those for the hot dwarfs have displacements to low v_phi that grow with |z| from very small values near the plane. At |z|>0.5 kpc, the theoretical v_phi distributions for giants show a deficit of stars with large v_phi and the model v_r distributions are too narrow. Systematically over-estimating distances by 20 per cent introduces asymmetry into the model v_r and v_z distributions near the plane and but significantly improves the fits to the data at |z|>0.5 kpc. The quality of the fits lends credence to the assumed, disc-dominated, gravitational potential.


The effect of phase front deformation on the growth of the filamentation instability in laser-plasma interactions

New Journal of Physics 15 (2013)

E Higson, PA Norreys, R Trines, R Bingham, KL Lancaster, J Jiang, JR Davies

Laser pulses of 0.9 kJ/1 ns/1053 nm were focused onto low-Z plastic targets in both spherical and planar geometry. The uniformity of the resulting plasma production was studied using x-ray pinhole imaging. Evidence is provided suggesting that thermal filamentation starts to occur for irradiances on the target of Iλ 10 W cm μm , even on deployment of phase plates to improve the focal spot spatial uniformity. The experiments are supported by both analytical modelling and two-dimensional particle-in-cell simulations. The implications for the applications of laser-plasma interactions that require high degrees of uniform irradiation are discussed. © IOP Publishing and Deutsche Physikalische Gesellschaft.


Stream-orbit misalignment II: A new algorithm to constrain the Galactic potential

ArXiv (2013)

JL Sanders, J Binney

In the first of these two papers we demonstrated that assuming streams delineate orbits can lead to order one errors in potential parameters for realistic Galactic potentials. Motivated by the need for an improvement on orbit-fitting, we now present an algorithm for constraining the Galactic potential using tidal streams without assuming that streams delineate orbits. This approach is independent of the progenitor mass so is valid for all observed tidal streams. The method makes heavy use of angle-action variables and seeks the potential which recovers the expected correlations in angle space. We demonstrate that the method can correctly recover the parameters of a simple two-parameter logarithmic potential by analysing an N-body simulation of a stream. We investigate the magnitude of the errors in observational data for which the method can still recover the correct potential and compare this to current and future errors in data. The errors in the observables of individual stars for current and near future data are shown to be too large for the direct use of this method, but when the data are averaged in bins on the sky, the resulting averaged data are accurate enough to constrain correctly the potential parameters for achievable observational errors. From pseudo-data with errors comparable to those that will be furnished in the era of Gaia (20 per cent distance errors, 1.2 mas/yr proper motion errors, and 10 km/s line-of-sight velocity errors) we recover the circular velocity, V_c=220 km/s, and the flattening of the potential, q=0.9, to be V_c=223+/-10km/s and q=0.91+/-0.09.


Multiscale Gyrokinetics for Rotating Tokamak Plasmas: Fluctuations, Transport and Energy Flows

ArXiv (2012)

IG Abel, GG Plunk, E Wang, M Barnes, SC Cowley, W Dorland, AA Schekochihin

This paper presents a complete theoretical framework for plasma turbulence and transport in tokamak plasmas. The fundamental scale separations present in plasma turbulence are codified as an asymptotic expansion in the ratio of the gyroradius to the equilibrium scale length. Proceeding order-by-order in this expansion, a framework for plasma turbulence is developed. It comprises an instantaneous equilibrium, the fluctuations driven by gradients in the equilibrium quantities, and the transport-timescale evolution of mean profiles of these quantities driven by the fluctuations. The equilibrium distribution functions are local Maxwellians with each flux surface rotating toroidally as a rigid body. The magnetic equillibrium is obtained from the Grad-Shafranov equation for a rotating plasma and the slow (resistive) evolution of the magnetic field is given by an evolution equation for the safety factor q. Large-scale deviations of the distribution function from a Maxwellian are given by neoclassical theory. The fluctuations are determined by the high-flow gyrokinetic equation, from which we derive the governing principle for gyrokinetic turbulence in tokamaks: the conservation and local cascade of free energy. Transport equations for the evolution of the mean density, temperature and flow velocity profiles are derived. These transport equations show how the neoclassical corrections and the fluctuations act back upon the mean profiles through fluxes and heating. The energy and entropy conservation laws for the mean profiles are derived. Total energy is conserved and there is no net turbulent heating. Entropy is produced by the action of fluxes flattening gradients, Ohmic heating, and the equilibration of mean temperatures. Finally, this framework is condensed, in the low-Mach-number limit, to a concise set of equations suitable for numerical implementation.


Measuring fast electron spectra and laser absorption in relativistic laser-solid interactions using differential bremsstrahlung photon detectors.

Rev Sci Instrum 84 (2013) 083505-

RH Scott, EL Clark, F Pérez, MJ Streeter, JR Davies, HP Schlenvoigt, JJ Santos, S Hulin, KL Lancaster, SD Baton, SJ Rose, PA Norreys

A photon detector suitable for the measurement of bremsstrahlung spectra generated in relativistically intense laser-solid interactions is described. The Monte Carlo techniques used to extract the fast electron spectrum and laser energy absorbed into forward-going fast electrons are detailed. A relativistically intense laser-solid experiment using frequency doubled laser light is used to demonstrate the effective operation of the detector. The experimental data were interpreted using the 3-spatial-dimension Monte Carlo code MCNPX [D. Pelowitz, MCNPX User's Manual Version 2.6.0, Los Alamos National Laboratory, 2008], and the fast electron temperature found to be 125 keV.


Universal behaviour of shock precursors in the presence of efficient cosmic ray acceleration

Monthly Notices of the Royal Astronomical Society 430 (2013) 2873-

B Reville, AR Bell


Dynamical models and Galaxy surveys

ArXiv (2013)

J Binney, JL Sanders

Equilibrium dynamical models are essential tools for extracting science from surveys of our Galaxy. We show how models can be tested with data from a survey before the survey's selection function has been determined. We illustrate the application of this method by presenting some results for the RAVE survey. We extend our published analytic distribution functions to include chemistry and fit the chosen functional form to a combination of the Geneva--Copenhagen survey (GCS) and a sample of G-dwarfs observed at z~1.75 kpc by the SEGUE survey. By including solid dynamics we are able to predict the contribution that the thick disc/halo stars surveyed by SEGUE should make to the GCS survey. We show that the measured [Fe/H] distribution from the GCS includes many fewer stars at [Fe/H]<-0.6 than are predicted. The problem is more likely to lie in discordant abundance scales than with incorrect dynamics.


DIFFUSE INTERSTELLAR BAND AT 8620 angstrom IN RAVE: A NEW METHOD FOR DETECTING THE DIFFUSE INTERSTELLAR BAND IN SPECTRA OF COOL STAR

ASTROPHYSICAL JOURNAL 778 (2013) ARTN 86

J Kos, T Zwitter, EK Grebel, O Bienayme, J Binney, J Bland-Hawthorn, KC Freeman, BK Gibson, G Gilmore, G Kordopatis, JF Navarro, Q Parker, WA Reid, G Seabroke, A Siebert, A Siviero, M Steinmetz, F Watson, RFG Wyse


High Mach-number collisionless shock driven by a laser with an external magnetic field

EPJ Web of Conferences 59 (2013)

T Morita, Y Sakawa, Y Kuramitsu, T Norimatsu, H Takabe, T Ide, K Tsubouchi, A Nishida, K Nishio, M Kuwada, H Ide, H Yoneda, T Namiki, K Tomita, K Nakayama, K Inoue, K Uchino, M Nakatsutsumi, A Pelka, M Koenig, Q Dong, D Yuan, G Gregori

Collisionless shocks are produced in counter-streaming plasmas with an external magnetic field. The shocks are generated due to an electrostatic field generated in counter-streaming laser-irradiated plasmas, as reported previously in a series of experiments without an external magnetic field [T. Morita et al., Phys. Plasmas, 17, 122702 (2010), Kuramitsu et al., Phys. Rev. Lett., 106, 175002 (2011)] via laser-irradiation of a double-CH-foil target. A magnetic field is applied to the region between two foils by putting an electro-magnet (∼10 T) perpendicular to the direction of plasma expansion. The generated shocks show different characteristics later in time (t > 20ns). © Owned by the authors, published by EDP Sciences, 2013.


Probing the complex ion structure in liquid carbon at 100 GPa

Physical Review Letters 111 (2013)

D Kraus, W Cayzac, A Ortner, A Otten, F Roth, G Schaumann, D Schumacher, K Siegenthaler, F Wagner, M Roth, J Vorberger, DO Gericke, K Wünsch, V Bagnoud, A Blažević, A Frank, G Gregori

We present the first direct experimental test of the complex ion structure in liquid carbon at pressures around 100 GPa, using spectrally resolved x-ray scattering from shock-compressed graphite samples. Our results confirm the structure predicted by ab initio quantum simulations and demonstrate the importance of chemical bonds at extreme conditions similar to those found in the interiors of giant planets. The evidence presented here thus provides a firmer ground for modeling the evolution and current structure of carbon-bearing icy giants like Neptune, Uranus, and a number of extrasolar planets. © 2013 American Physical Society.


Stream-orbit misalignment I: The dangers of orbit-fitting

ArXiv (2013)

JL Sanders, J Binney

Tidal streams don't, in general, delineate orbits. A stream-orbit misalignment is expected to lead to biases when using orbit-fitting to constrain models for the Galactic potential. In this first of two papers we discuss the expected magnitude of the misalignment and the resulting dangers of using orbit-fitting algorithms to constrain the potential. We summarize data for known streams which should prove useful for constraining the Galactic potential, and compute their actions in a realistic Galactic potential. We go on to discuss the formation of tidal streams in angle-action space, and explain why, in general, streams do not delineate orbits. The magnitude of the stream-orbit misalignment is quantified for a logarithmic potential and a multi-component Galactic potential. Specifically, we focus on the expected misalignment for the known streams. By introducing a two-parameter family of realistic Galactic potentials we demonstrate that assuming these streams delineate orbits can lead to order one errors in the halo flattening and halo-to-disc force ratio at the Sun. We present a discussion of the dependence of these results on the progenitor mass, and demonstrate that the misalignment is mass-independent for the range of masses of observed streams. Hence, orbit-fitting does not yield better constraints on the potential if one uses narrower, lower-mass streams.


Visualizing electromagnetic fields in laser-produced counter-streaming plasma experiments for collisionless shock laboratory astrophysics

Physics of Plasmas 20 (2013)

NL Kugland, JS Ross, SH Glenzer, C Huntington, MC Levy, D Martinez, C Plechaty, BA Remington, DD Ryutov, H-S Park, P-Y Chang, G Fiksel, DH Froula, RP Drake, M Grosskopf, C Kuranz, G Gregori, J Meinecke, B Reville, M Koenig, A Pelka, A Ravasio, Y Kuramitsu, T Morita, Y Sakawa, H Takabe, E Liang, F Miniati, R Presura, A Spitkovsky

Collisionless shocks are often observed in fast-moving astrophysical plasmas, formed by non-classical viscosity that is believed to originate from collective electromagnetic fields driven by kinetic plasma instabilities. However, the development of small-scale plasma processes into large-scale structures, such as a collisionless shock, is not well understood. It is also unknown to what extent collisionless shocks contain macroscopic fields with a long coherence length. For these reasons, it is valuable to explore collisionless shock formation, including the growth and self-organization of fields, in laboratory plasmas. The experimental results presented here show at a glance with proton imaging how macroscopic fields can emerge from a system of supersonic counter-streaming plasmas produced at the OMEGA EP laser. Interpretation of these results, plans for additional measurements, and the difficulty of achieving truly collisionless conditions are discussed. Future experiments at the National Ignition Facility are expected to create fully formed collisionless shocks in plasmas with no pre-imposed magnetic field. © 2013 AIP Publishing LLC.


Surface waves and electron acceleration from high-power, kilojoule-class laser interactions with underdense plasma

New Journal of Physics 15 (2013)

L Willingale, AGR Thomas, A Maksimchuk, C Zulick, K Krushelnick, PM Nilson, RS Craxton, TC Sangster, C Stoeckl, H Chen, J Cobble, PA Norreys, RHH Scott

Experiments were performed on the Omega EP laser facility to study laser pulse propagation, channeling phenomena and electron acceleration from high-intensity, high-power laser interactions with underdense plasma. A CH plasma plume was used as the underdense target and the interaction of the laser pulse channeling through the plasma was imaged using proton radiography. High-energy electron spectra were measured for different experimental laser parameters. Structures observed along the channel walls are interpreted as having developed from surface waves, which are likely to serve as an injection mechanism of electrons into the cavitated channel for acceleration via direct laser acceleration mechanisms. Two-dimensional particle-in-cell simulations give good agreement with these channeling and electron acceleration phenomena. © IOP Publishing and Deutsche Physikalische Gesellschaft.


Implosion and heating experiments of fast ignition targets by Gekko-XII and LFEX lasers

EPJ Web of Conferences 59 (2013)

H Shiraga, S Fujioka, M Nakai, T Watari, H Nakamura, Y Arikawa, H Hosoda, T Nagai, M Koga, H Kikuchi, Y Ishii, T Sogo, K Shigemori, H Nishimura, Z Zhang, M Tanabe, S Ohira, Y Fujii, T Namimoto, Y Sakawa, O Maegawa, T Iwawaki, K Shimada, H Nagatomo, M Murakami, T Norimatsu, H Homma, Y Fujimoto, N Miyanaga, J Kawanaka, T Jitsuno, Y Nakata, K Tsubakimoto, N Morio, S Matsuo, T Kawasaki, K Sawai, K Tsuji, H Murakami, N Sarukura, T Shimizu, K Mima, H Azechi, T Ozaki, H Sakagami, A Iwamoto, KA Tanaka, H Habara, K Sueda, R Kodama, M Key, P Norreys, J Pasley, T Johzaki, A Sunahara, T Taguchi, T Kanabe, K Kondo

The FIREX-1 project, the goal of which is to demonstrate fuel heating up to 5 keV by fast ignition scheme, has been carried out since 2003 including construction and tuning of LFEX laser and integrated experiments. Implosion and heating experiment of Fast Ignition targets have been performed since 2009 with Gekko-XII and LFEX lasers. A deuterated polystyrene shell target was imploded with the 0.53- μm Gekko-XII, and the 1.053- μm beam of the LFEX laser was injected through a gold cone attached to the shell to generate hot electrons to heat the imploded fuel plasma. Pulse contrast ratio of the LFEX beam was significantly improved. Also a variety of plasma diagnostic instruments were developed to be compatible with harsh environment of intense hard x-rays (γ rays) and electromagnetic pulses due to the intense LFEX beam on the target. Large background signals around the DD neutron signal in time-of-flight record of neutron detector were found to consist of neutrons via (γ,n) reactions and scattered gamma rays. Enhanced neutron yield was confirmed by carefully eliminating such backgrounds. Neutron enhancement up to 3.5 × 10 was observed. Heating efficiency was estimated to be 10-20% assuming a uniform temperature rise model. © Owned by the authors, published by EDP Sciences, 2013.


Pair plasma cushions in the hole-boring scenario

Plasma Physics and Controlled Fusion 55 (2013)

JG Kirk, AR Bell, CP Ridgers

Pulses from a 10 PW laser are predicted to produce large numbers of gamma-rays and electron-positron pairs on hitting a solid target. However, a pair plasma, if it accumulates in front of the target, may partially shield it from the pulse. Using stationary, one-dimensional solutions of the two-fluid (electron-positron) and Maxwell equations, including a classical radiation reaction term, we examine this effect in the hole-boring scenario. We find the collective effects of a pair plasma 'cushion' substantially reduce the reflectivity, converting the absorbed flux into high-energy gamma-rays. There is also a modest increase in the laser intensity needed to achieve threshold for a non-linear pair cascade. © 2013 IOP Publishing Ltd.


Impact of extended preplasma on energy coupling in kilojoule energy relativistic laser interaction with cone wire targets relevant to fast ignition

New Journal of Physics 15 (2013)

T Yabuuchi, R Mishra, C McGuffey, B Qiao, H Sawada, T Ma, DP Higginson, FN Beg, MS Wei, KU Akli, RB Stephens, Y Sentoku, H Chen, MH Key, AJ MacKinnon, HS McLean, PK Patel, Y Ping, D Batani, LA Gizzi, PA Norreys, W Theobald, C Stoeckl

Cone-guided fast ignition laser fusion depends critically on details of the interaction of an intense laser pulse with the inside tip of a cone. Generation of relativistic electrons in the laser plasma interaction (LPI) with a gold cone and their subsequent transport into a copper wire have been studied using a kJ-class intense laser pulse, OMEGA EP (850 J, 10 ps). Weobserved that the laser-pulse-energy-normalized copper K signal from the Cu wire attached to the Au cone is significantly reduced (by a factor of 5) as compared to that from identical targets using the Titan laser (150 J, 0.7 ps) with 60 × less energy in the prepulse. We conclude that the decreased coupling is due to increased prepulse energy rather than 10 ps pulse duration, for which this effect has not been previously explored. The collisional particle-in-cell code PICLS demonstrates that the preformed plasma has a significant impact on generation of electrons and their transport. In particular, a longer scale length preplasma significantly reduces the energy coupling from the intense laser to the wire due to the larger offset distance between the relativistic critical density surface and the cone tip as well as a wider divergence of source electrons. We also observed that laser-driven plasma ionization increase in the LPI region can potentially alter the electron density profile during the laser interaction, forcing the electron source to be moved farther away from the cone tip which contributes to the reduction of energy coupling. © IOP Publishing and Deutsche Physikalische Gesellschaft.